The evolution of uterine activity for labor requires obligatory changes in membrane function including enhanced excitability and reactivity, facilitated electrocoupling via low impedance pathways (gap junctions) and efficient excitation-contraction coupling. The rhythmically contracting myometrial cells deform an elastic connective tissue matrix to generatie uterine wall tension. The properties of the sarcolemma and lysosomal membranes are controlled in part by steroid hormone actions but little is known of the mechanisms of post-nuclear expression of these regulators. We have developed methods enzbling high purification of the sarcolemma and others enabling specific manipulation of membrane function by steroid treatment in organ culture. These new methods will be combined with in situ studies of the control of excitability, electropropagation and uterine activity in sheep and laboratory animals undergoing physiological manipulation of pregnancy. All phenomena observed in vivo will be examined in detail using the new in vitro methodology and related techniques developed by other laboratories.